This application is related to, and claims priority to, Japanese Application No. JP 2002-219217 filed Jul. 29, 2002, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a printed wiring board for surface mounting electronic parts, and, more particularly, to a multilayered printed wiring board having insulator layers made of a glass cloth and an epoxy resin.
2. Description on the Related Art
Examples of a printed wiring board for surface mounting electronic parts include a copper-clad paper phenol laminate referred to as FR-1, a copper-clad composite laminate referred to as CEM-3, a resin sheet with a copper foil referred to as RSC used in layering board, a multilayered copper-clad glass epoxy laminate referred to as FR-4 according to the NEMA (National Electrical Manufacturers Association, U.S.) Standard, etc. Of these examples, the multilayered copper-clad glass epoxy laminate referred to as FR-4 is a multilayered printed wiring board having insulator layers made of glass cloth and epoxy resin. The wiring boards have become a main component mounted in such apparatuses as personal computers, cellular phones, etc.
The temperature of the printed wiring board becomes high in some electric circuits. Thus, fire retardant materials are required to prevent fires. Standards of fire retardancy exist, such as UL94-V0.
Halogen has been used to make printed wiring boards fire retardant. Fire retardant components such as halogen are generally added to or made to react with resin. Such fire retardant additives provide high fire retardancy and generate halogenated radicals, mainly due to pyrolysis, which capture inorganic radicals that are combustion sources to stop chain reactions resulting in combustion.
In the copper-clad glass epoxy laminate like the above-described FR-4, an insulator layer is made of glass cloth and epoxy resin, such as chlorinated epoxy resin or brominated epoxy resin.
However, a fire retardant additive that includes a large amount of halogen compound, such as brominated epoxy resin, may generate dioxin under certain combustion conditions. Therefore, because of environmental protection concerns, there is a need to reduce the amount of halogen. In addition, another problem exists in that liberated halogen ions can migrate into the copper foil layer, causing a phenomenon referred to as migration that induces corrosion of the copper foil layer, on which a circuit is formed.
Fire retardant board material with a reduced amount of halogen is disclosed in, for example, Japanese Patent Application Publication No. JP-2001-164094, in which a fire retardant insulating epoxy composition is used as an insulator film for layers, and provides fire retardancy and insulation characteristics required for a circuit board. The composition includes epoxy resin, a curing agent for the epoxy resin, resorcinol phosphate ester, aluminum hydroxide, and an epoxy curing agent.
Also, in Japanese Patent Application Publication No. JP-2000-290474, a hardening resin composition having sufficient fire retardancy characteristics is disclosed that contains polyphenylene ether resin, bisphenol A epoxy resin, novolak phenolic resin, melamine or guanamine, phosphorus compound, and inorganic filler.
In the compositions disclosed in the above-described JP-2001-164094 and JP-2000-290474, fire retardancy is provided by inorganic components such as phosphorus compound, aluminum hydroxide, etc., which enables the reduction of halogen.
However, in the fire retardant insulating epoxy composition disclosed in JP-2001-164094, a layered printed wiring board is made without using glass cloth. Thus, the composition cannot be directly applied to the multilayered copper-clad glass epoxy laminate. Moreover, the combination of phosphorus ester and aluminum hydroxide makes it difficult to obtain the fire retardancy equivalent to that in UL-V0. Thus, fire retardancy may be insufficient in the composition disclosed in JP-2001-164094.
In addition, in the resin composition disclosed in JP-2000-290474, the composition and mixing amounts of the inorganic fillers are not limited and, thus, insufficient fire retardancy may result.
Furthermore, in each of the compositions disclosed in the above-described JP-2001-164094 and JP-2000-290474, a problem exists of providing low thermal conductivity of the insulator layer. In general, inorganic fillers such as aluminum hydroxide may cause secondary flocculation on the order of 10 to 20 xcexcm. The flocculation is also caused when inorganic fillers are distributed in the resin. Thus, it is difficult to uniformly distribute inorganic fillers in the insulator layer.
Consequently, in the above-described conventional compositions, some portions of the insulator layer have been formed rich in resin. This reduces thermal conductivity of the insulator layer to cause insufficient dissipation of heat when electronic parts are soldered by, for example, a reflow furnace. Thus, electronic parts may be damaged.
Further, for the multilayered printed wiring board, recent downsizing and performance improvement thereof have caused the operating conditions to become severe. Thus, improvement in manufacturing reliability and thermal properties of the board are required. For example, through-holes are provided for continuity between metal foil layers laminated alternately with insulator layers. The ability to drill the through-holes through the insulator layers may be reduced when the amount of added inorganic fillers is increased to ensure fire retardancy. Therefore, improved drilling machinability is needed when adding inorganic fillers such as those described above.
It is an aspect of the present invention to provide a fire retardant multilayered printed wiring board that reduces the amounts of halogen compounds used, with enhanced thermal conductivity of the insulator layers to prevent electronic components from being damaged during soldering, and that has excellent drilling machinability for forming through-holes.
Additional aspects and advantages of the invention will be set forth in part in the description that follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and/or other aspects according to the present invention, there is provided a multilayered printed wiring board having insulator layers that are formed with a glass cloth impregnated with an epoxy resin composition, and metal foil layers in which a circuit is formed. The insulator layers are laminated alternately with the metal foil layers so that each of the insulator layers is sandwiched between the two metal layers. Through-holes are formed to provide continuity between the metal foil layers laminated with the insulator layers placed therebetween. The epoxy resin composition includes at least phosphate ester, aluminum hydroxide, silica, calcium oxide, strontium titanate, and iron oxide. Amounts of inorganic components in the insulator are provided in the ranges of about 8% to about 18% by mass of phosphorus pentoxide (P2O5), about 20% to about 28% by mass of aluminum oxide (Al2O3), about 0.1% to about 0.6% by mass of silica (SiO2), about 0.1% to about 1.0% by mass of chlorine (Cl), about 15% to about 20% by mass of calcium oxide (CaO), about 0.3% to about 0.5% by mass of titanium oxide (TiO2), about 0.2% to about 0.4% by mass of iron oxide (Fe2O3), and about 0.1% to about 0.3% by mass of strontium oxide (SrO), in converted values as measured using X-ray fluorescence analysis.
Phosphate ester and aluminum hydroxide in the amounts as described above provide fire retardancy while reducing the amounts of halogen compounds used.
Silica, calcium oxide, strontium titanate, and iron oxide are contained in the above-described proportions to prevent secondary flocculation of aluminum hydroxide in the insulator layers and to enhance thermal conductivity of the insulator layers, which can prevent electronic components from being damaged during soldering.
Because the amounts of the inorganic components used are low, drilling machinability in forming through-holes is not reduced.
The aluminum hydroxide component contains particles with irregular shapes having an average particle diameter of about 0.5 xcexcm to about 1.0 xcexcm that are approximately uniformly distributed in the insulator layers. This prevents the aluminum hydroxide particles from causing secondary flocculation to further enhance thermal conductivity of the insulator layer.
At specified sections in the metal foil layers and through-holes, solder layers are formed that do not contain lead. Thus, the multilayered printed wiring board according to aspects of the present invention is lead-free in addition to having less halogen, which reduces environmental pollution.
A surface of the multilayered printed wiring board, except for sections to be soldered, is coated with an insulation material containing phthalocyanine blue. This removes halogen from the resist coat material on the surface of the wiring board, which reduces the amount of environmental pollution.
Amounts of gases evolved by a burning test at 600xc2x0 C., as specified under JIS-K7217, are about 150 xcexcg/g or less of chlorine, about 30 xcexcg/g or less of bromine, about 5 mg/g or less of hydrogen cyanide, and about 5 pg/g or less of polychlorinated dibenzo-p-dioxin and polychlorinated dibenzo furan in mass ratio to the multilayered printed wiring board. This suppresses the production of injurious halogen and dioxin during soldering.
Also, thermal conductivity of the insulator layers is about 0.4 to 0.5 W/m/K. The high thermal conductivity of the insulator layers prevents electronic components from being damaged during soldering.